1. Field of the Invention
The invention is related to the field of nuclear magnetic resonance ("NMR") sensing apparatus and measuring techniques. More specifically, the invention is related to NMR well logging apparatus and measuring techniques for sensing within earth formations penetrated by a wellbore. The invention also relates to methods for using NMR measurements to determine properties of the earth formations.
2. Description of the Related Art
NMR well logging instruments can be used for determining properties of earth formations, including the fractional volume of pore space ("porosity"), the fractional volume of mobile fluid filling the pore spaces of the earth formations and other petrophysical parameters. Examples of methods and measurement techniques for using NMR measurements for determining the fractional volume of pore space, the fractional volume of mobile fluid and other petrophysical parameters are described in, Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination, M. N. Miller et al, Society of Petroleum Engineers paper no. 20561, Richardson, Tex. (1990) and in, Field Test of an Experimental Pulsed Nuclear Magnetism Tool, C. E. Morriss et al, SPWLA Logging Symposium Transactions, paper GGG (1993).
NMR well logging instruments typically include a permanent magnet to induce a static magnetic field within the earth formations and include a transmitting antenna positioned near the magnet and shaped so that a pulse of radio frequency ("RF") power conducted through the antenna induces an RF magnetic field in the earth formations. The RF magnetic field is generally orthogonal to the static magnetic field. After an RF pulse, voltages are induced in a receiving antenna on the logging instrument by precessional rotation of spin axes of hydrogen or other nuclei about the static magnetic field. The receiving antenna is typically connected to a receiver circuit in the instrument which detects and measures the induced voltages. In a typical NMR measurement set a sequence of RF pulses is applied to the transmitting antenna and a sequence of voltages is measured by the receiving antenna (note that some instruments use the same antenna for transmitting and receiving). The magnitude of the detected voltages and the rates at which the detected voltages vary with time are related to certain petrophysical properties of the earth formation.
One type of NMR well logging apparatus is described, for example, in U.S. Pat. No. 3,597,681 issued to Huckbay et al. The apparatus disclosed in the Huckbay et al '681 patent has several drawbacks, one of which is that a region of unidirectional static magnetic field is not homogeneous along the wellbore axis. As a practical matter, well logging instruments typically must be able to move axially through the wellbore while making measurements. During the time needed to make a typical NMR measurement, the "sensitive volume" (that part of the formation in which nuclear magnetic resonance is excited) generated by the logging instrument will be moved through the wellbore so that the measurement set cannot be completed. Another drawback to the apparatus disclosed in the Huckbay et al '681 patent is that a significant part of the NMR signals originate from within the fluid filling the wellbore (called "drilling mud").
Yet another drawback to the apparatus disclosed in the Huckbay et al '681 patent is that its antenna is directed to one side of the apparatus and therefore uses only a small fraction of the total volume of unidirectional static magnetic field. This results in an inefficient use of the permanent magnet in the instrument.
Still another drawback to the apparatus disclosed in the Huckbay et al '681 patent is that the antenna is subject to a high static magnetic field strength and, therefore, can have an unacceptably high amount of magnetoacoustic ringing.
Another drawback to the apparatus disclosed in the Huckbay et al '681 patent is that the RF magnetic field generated by the antenna drops in magnitude as the third power of the distance from the instrument to the sensitive volume since the antenna in this instrument is the equivalent of a three dimensional magnetic dipole. Such an antenna is proximally coupled only to a small part of the unidirectional static magnetic field. This results in an extremely low signal-to-noise ratio.
Another type of NMR well logging instrument is described in U.S. Pat. No. 4,350,955 issued to Jackson et al. The instrument disclosed in the Jackson et al '955 patent includes permanent magnets configured to induce a magnetic field in the earth formations which has a toroidal volume of substantially uniform magnetic field strength. A particular drawback to the apparatus disclosed in the Jackson et al '955 patent is that the thickness of the toroidal volume is very small relative to typical rates of axial motion of well logging tools during measurement operations. Well logging instruments, in order to be commercially useful, typically must be able to move axially through the wellbore at rates not less than about ten feet per minute. The length of time needed to make a typical NMR spin-echo measurement set can be as long as several seconds. The NMR logging instrument is therefore likely to move a substantial distance during a measurement cycle. Measurements made by the instrument disclosed in the Jackson et al '955 patent are therefore subject to error as the instrument is moved during logging operations, because the antenna would no longer be positioned so as to be sensitive to the same toroidal volume which was magnetized at the beginning of any measurement set.
Another drawback to the apparatus instrument in the Jackson et al '955 patent is that it does not eliminate NMR signals originating within the fluid filling the wellbore.
A still further drawback to the apparatus disclosed in the Jackson et al '955 patent is that the toroidally shaped static magnetic field is can change in amplitude as the instrument is subjected to changes in ambient temperature and variances in the earth's magnetic field. The antenna in the Jackson et al '955 apparatus is tuned to a single frequency. If the field strength of the static magnetic field in the toroidal volume changes, the antenna may no longer be sensitive to NMR signals originating within the toroidal volume. Using the apparatus in Jackson et al '955, it is impractical to compensate the frequency of the RF magnetic field for changes in the static magnetic field strength within the toroidal volume.
Additional drawbacks to the apparatus disclosed in the Jackson et al '955 patent include the magnet pole pieces being opposed each other. This results in a significant demagnetizing effect which requires magnet material having a high coercive force. This requirement is in directly opposed to the requirement for strong residual magnetization and high temperature stability of the permanent magnet. Second, the magnet pole pieces are spaced apart and are far away from the toroidal region, which makes the use of the permanent magnet material less efficient. Third, the antenna used in the Jackson '955 apparatus has low efficiency as a result of low electromagnetic coupling between the antenna and the earth formation at the resonant frequency for NMR experimentation. Fourth, the antenna is located in a relatively strong static magnetic field, which stimulates magnetoacoustic ringing in the antenna. Fifth, for an NMR measurement technique which uses a homogeneous static magnetic field, changes in the relative position of the instrument with respect to the earth's magnetic field can cause a significant disturbance to the homogeneity of the toroidal region.
Another type of NMR well logging apparatus is described in U.S. Pat. No. 4,717,876 issued to Masi et al. The apparatus disclosed in the Masi et al '876 patent has improved homogeneity in the toroidal region as compared to the apparatus described in the Jackson et al '955 patent, but has basically the same drawbacks as the Jacskon et al '955 apparatus.
Another type of NMR well logging apparatus is described in U.S. Pat. No. 4,629,986 issued to Clow et al. This apparatus provides improved signal-to-noise ratio compared with the apparatus of Jackson et al '955 by including a high magnetic permeability ferrite in the antenna. Increased stability is achieved by performing the NMR measurements in a static magnetic field which includes an amplitude gradient. However, the apparatus disclosed in the Clow et al '986 patent has several drawbacks. Since the magnetic properties of most permanent magnet materials are temperature dependent, the sensitive volume is not stable in shape and magnetic field intensity. The sensitive volume of this instrument is only a couple of inches long in the longitudinal direction, which requires that this instrument be practically stationary during an NMR measurement cycle. The magnet pole pieces are substantially spaced apart and are far from the sensitive region, which makes the use of the permanent magnet material inefficient. The antenna is located in a relatively strong magnetic field, which stimulates magnetoacoustic ringing in the antenna. The high magnetic permeability ferrite in the antenna is located in a relatively strong magnetic field, which may saturate the ferrite and reduce its efficiency. Soft ferrite disposed in a static magnetic field is also a strong source of magnetostrictive ringing following any RF pulse through the antenna. In the magnet arrangement of the Clow et al '986 patent, the demagnetizing field is relatively strong, which requires a magnet material having high coercive force. This requirement is opposite to the strong residual magnetization and high temperature stability of the magnetic properties also required of the permanent magnet material. Finally, the static magnetic field in the earth formations in the sensitive volume is only about 10 Gauss and rotates 360.degree. in a plane perpendicular to the wellbore axis. For this amplitude of static magnetic field, the earth's magnetic field amplitude of about 0.5 Gauss presents a significant disturbance to the overall field strength.
Another type of NMR well logging apparatus described in U.S. Pat. No. 4,717,878 issued to Taicher et al provides azimuthal resolution with respect to the wellbore axis and reduction of spurious signals from the wellbore fluid. However, the apparatus disclosed in the Taicher et al '878 patent has several drawbacks. Since the magnetic properties of the permanent magnet material used in this apparatus are temperature dependent, the sensitive region does not have a stable in shape or stable magnetic field intensity. The antenna is located within a relatively strong magnetic field, which stimulates magnetoacoustic ringing in the antenna. In the arrangement of the magnet in the apparatus disclosed in the Taicher et al '878 patent, the demagnetizing field is very strong, which requires a magnet material having high coercive force. This requirement is directly opposite to the strong residual magnetization and high temperature stability of magnetic properties required of the permanent magnet for a well logging apparatus.
Due to the disadvantages of the foregoing NMR well logging instrument designs, none of them are generally commercially accepted well logging instruments. Commercially accepted well logging instruments include one which is described in U.S. Pat. No. 4,710,713 issued to Taicher et al. The instrument disclosed in the Taicher et al '713 patent includes a generally cylindrical permanent magnet assembly which induces a static magnetic field having substantially uniform magnetic field strength within an annular cylindrical volume in the earth formations. The instrument disclosed in the Taicher et al '713 patent has several drawbacks, however. First, the antenna induces an RF magnetic field within the earth formations surrounding the tool which decreases in strength as the square of the radial distance from the magnet. Because the signal-to-noise ratio of NMR measurements made within a gradient magnetic field is typically related to the strength of the RF magnetic field, the apparatus disclosed in the Taicher et al '713 has very high power requirements, and can have difficulty obtaining measurements having sufficient signal-to-noise ratio at substantial radial distances from the instrument.
Another drawback to the instrument of the Taicher et al '713 patent is that the optimum design of the magnet and the RF antenna, for purposes of optimizg the signal-to-noise ratio, requires that nuclear magnetic resonance conditions be met at a relatively high frequency. Since the RF energy losses in the electrically conductive fluid in the wellbore are proportional to the square of the frequency, the operation of the Taicher et al '713 patent is restricted to use in relatively low conductivity fluids in the wellbore.
Yet another drawback to the apparatus of the Taicher et al '713 patent is that the antenna is located within a relatively strong static magnetic field which is perpendicular to a direction of RF current flow in the transmitting antenna and, therefore, stimulates magnetoacoustic ringing in the transmitting antenna.
Another NMR logging instrument is described in U.S. Pat. No. 5,055,787 issued to Kleinberg et al. This logging instrument includes permanent magnets arranged to induce a magnetic field in the earth formation having substantially zero field gradient within a predetermined sensitive volume. The magnets are arranged in a portion of the tool housing which is typically placed in contact with the wall of the wellbore. The antenna in this instrument is positioned in a recess located external to the tool housing, enabling the tool housing to be constructed of high strength material such as steel. A drawback to the logging instrument in the Kleinberg et al '787 patent is that its sensitive volume is only about 0.8 cm away from the tool surface and extends only to about 2.5 cm radially outward from the tool surface. Measurements made by this instrument are therefore subject to large error caused by, among other things, roughness in the wall of the wellbore, by deposits of the solid phase of the drilling mud (called "mudcake") onto the wall of the wellbore in any substantial thickness, and by the fluid content of the formation in the invaded zone.
Another drawback to the instrument disclosed in the Kleinberg et al '787 patent relates to the permanent magnet material. Since the magnet pole pieces are opposed each other, there is a strong demagnetizing effect which requires a permanent magnet material having high coercive force. This requirement is opposite to the strong residual magnetization and high temperature stability of magnetic properties required of the permanent magnet.
Another NMR measurement apparatus which may have application for well logging is disclosed in U.S. Pat. No. 5,572,132 issued to Pulyer et al. This apparatus includes a permanent magnet for inducing a magnetic field polarized along the longitudinal axis of the apparatus, and antenna coils disposed about the exterior of the magnet. The apparatus described in the Pulyer et al '132 patent, as do most prior art NMR well logging instruments, has a common drawback which is explained, for example, in U.S. Pat. No. 5,332,967 issued to Shporer. This drawback is related to a significant phase shift of the NMR signal, which leads to significant distortion of the NMR signal height and may even lead to a complete disappearance of the NMR signal, when the logging apparatus is moving in a direction along a static magnetic field amplitude gradient. In actual well logging practice, the phase shift and signal reduction may be even worse than is suggested by the Shporer '967 patent because the logging speed can be variable, as is understood by those skilled in the art of well logging.